Fuel cell technology shows great promise as an alternative energy source for numerous applications. Several types of fuel cells have been constructed, including polymer electrolyte membrane fuel cells, direct methanol fuel cells, alkaline fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, and solid oxide fuel cells. For a comparison of several fuel cell technologies, see Los Alamos National Laboratory monograph LA-UR-99-3231 entitled Fuel Cells: Green Power by Sharon Thomas and Marcia Zalbowitz.
FIG. 1 represents an example of a fuel cell 100, including a high surface area anode 110 including an anode catalyst 112, a high surface area cathode 120 including a cathode catalyst 122, and an electrolyte 130 between the anode and the cathode. The electrolyte may be a liquid electrolyte; it may be a solid electrolyte, such as a polymer electrolyte membrane (PEM); or it may be a liquid electrolyte contained within a host material, such as the electrolyte in a phosphoric acid fuel cell (PAFC).
In operation of the fuel cell 100, fuel in the gas and/or liquid phase is brought over the anode 110 where it is oxidized at the anode catalyst 112 to produce protons and electrons in the case of hydrogen fuel, or protons, electrons, and carbon dioxide in the case of an organic fuel. The electrons flow through an external circuit 140 to the cathode 120 where air, oxygen, or an aqueous oxidant (e.g., peroxide) is being fed. Protons produced at the anode 110 travel through electrolyte 130 to cathode 120, where oxygen is reduced in the presence of protons and electrons at cathode catalyst 122, producing water in the liquid and/or vapor state, depending on the operating temperature and conditions of the fuel cell.
Hydrogen and methanol have emerged as important fuels for fuel cells, particularly in mobile power (low energy) and transportation applications. The electrochemical half reactions for a hydrogen fuel cell are listed below.
Anode:2H2→4 H+ + 4 e−Cathode:O2 + 4 H+ + 4 e−→2 H2OCell Reaction:2 H2 + O2→2 H2OTo avoid storage and transportation of hydrogen gas, the hydrogen can be produced by reformation of conventional hydrocarbon fuels. In contrast, direct liquid fuel cells (DLFCs) utilize liquid fuel directly, and do not require a preliminary reformation step of the fuel. As an example, the electrochemical half reactions for a Direct Methanol Fuel Cell (DMFC) are listed below.
Anode:CH3OH + H2O→CO2 + 6 H+ + 6 e−Cathode:1.5 O2 + 6 H+ + 6 e−→3 H2OCell Reaction:CH3OH + 1.5 O2→CO2 + 2 H2O
One challenge faced in developing DLFCs is the minimization of fuel “crossover.” The material used to separate the liquid fuel feed from the gaseous oxidant feed in a DLFC typically is a stationary PEM that is not fully impermeable to fuels, such as methanol. As a result, fuel may cross over the membrane from the anode to the cathode, reacting with the cathode catalyst directly in the presence of oxygen to produce heat, water and carbon dioxide but no useable electric current. In addition to being a waste of fuel, crossover causes depolarization losses due to a mixed potential at the cathode and, in general, leads to decreased cell performance. Prior attempts to inhibit undesirable fuel crossover have met with mixed success. Measures that block migration of fuel to the cathode also typically hinder the flow of protons to the cathode, resulting in resistive losses in the fuel cell.
It is desirable to provide a system for minimizing fuel crossover in a fuel cell, while maintaining acceptable levels of proton transport to the cathode. Preferably, such a system also would provide one or more additional benefits, such as minimization of water accumulation at the cathode (referred to as “cathode flooding”) or tunable fuel cell performance.